1. Field of the Invention
The invention is concerned with controlling a hydraulic motor which raises and lowers relatively heavy loads. Particularly, the invention is concerned with such a system which includes a safety feature wherein if any of the hydraulic lines of the system fail while the load is in a raised position, the load is prevented from falling at a very fast rate by particularly improved check valve means arranged in the end of the hydraulic motor. Still more particularly, the invention is concerned with a hydraulic motor control system as discussed above which provides a Float position of operation wherein both the head end and rod end of the motor communicate with drain at the same time. Such a Float position is particularly useful with front loading loaders so that as a bucket thereof is pushed across uneven ground the bucket will raise and lower with the contour of the ground. A Float position is likewise desirable for the same reason with dozers. While the improvement of the present invention is particularly useful with front loading loaders and dozers it should be noted that it is also useful with a number of other apparatus, for example tractors, scrappers, lift-trucks and the like.
2. Prior Art
Fluid-driven motors are usually controlled with a directional control valve wherein a spool or other valve member is shiftable between a Neutral (or Hold) position at which the motor is stopped, another position, e.g., a Raise position for operating the motor in a first direction, and yet another position, e.g., a Lower position, for reversing operation of the motor. In many instances, the motor moves a load which may occasionally tend to move faster than at the rate which is provided for by the rate at which fluid is supplied to the motor in the directional control valve.
For example, power loaders as are used to handle loose earth or other bulk material have a bucket carried on a pair of pivoting lift-arms at the front of the vehicle where an upward and downward movement of the bucket is typically provided for by fluid motors controlled by the vehicle operator. When the loaded bucket is being lowered, gravitational force may tend to drive the fluid motors faster than is provided for by the supply of driving fluid. Under this condition, motor cavitation will occur, with well-known undesirable effects, unless corrective means are provided. One such means of preventing cavitation are makeup valves which sense incipient cavitation and open to supplement the driving fluid to the motor with fluid which is being discharged from the motor.
Providing of a Float position in the control system may also be useful. For example, a loader as discussed above is often used to pick up loose material by pushing the bucket along the surface of the ground to receive such material. On an uneven terrain, it is desirable that the bucket follow the contour of the ground and a Float position of the control system enables the bucket to do this.
A quite serious problem can arise if the hydraulic lines leading from the pump to the fluid driven motors should break. In such a case, when heavy loads are in the Raise position these loads would then tend to fall very rapidly, thus potentially causing significant damage to the apparatus as well as to the load itself. Thus, it is quite desirable for safety considerations to provide some means for checking the rate of fall of a load from the Raise position in the event of line failure. However, any such solution must at the same time allow for the providing of the aforementioned Float position with its appropriate connections as well as for operator controlled lowering of the load, i.e., a Lower position. It should also be noted that it is highly desirable that if the hydraulic lines to the hydraulic motor should break then the means which provides a reduced lowering rate is still operative.